During the summer monsoon the upper troposphere over South Asia is characterized by the monsoon anticyclone centered above the Tibetan Plateau. Surface air that has been rapidly transported upwards through deep convection becomes trapped within the strong anticyclonic circulation. Observations of trace gases within this anticyclone by the CARIBIC flying observatory revealed large enhancements in the greenhouse gas methane (CH4), which increased over the course of the monsoon. Meteorological analysis indicated that these air masses originated primarily in India, for which relatively little is known about CH4 emissions. Using correlations between concentrations of CH4 and carbon monoxide (CO) we estimated total emissions of 30.8 Tg CH4during the 2008 monsoon season (June–September), 19.7 Tg of which were identified as additional, monsoon-related biogenic methane using the relationship of CH4 to ethane (C2H6). After accounting for the ∼3.9 Tg attributed to rice agriculture in the current inventories, ∼15.8 Tg of additional CH4 remain. Underestimated rice emissions provide a partial explanation, with the remainder most likely attributable to microbial production in waterlogged areas such as landfills, polluted waterways and wetlands.